Earlier versions of the malware stemmed from a pop-up window that tricked users into installing a fake version of Adobe Flash. Newer versions exploited a security flaw in Java software that infected Macs by redirecting users to a bogus site. Once hackers gained control of the computers, they could spy on Mac users and steal their personal and banking data.

Security experts have called Flashback the largest and most sophisticated attack on Macs to date. And many have criticized Apple for not patching the Java security flaw, which had been publicly identified in February.

Apple also said Tuesday that it was working with Internet service providers worldwide to disable the computer servers hosted by the malware's creators.

Experts say the Flashback virus should serve as a wake-up call to Mac users whose computers are often considered more secure than PCs running Microsoft Windows. A 2009 survey by the antivirus firm ESET found that more than half of Americans considered PCs to be "very" or "extremely" vulnerable to cyberattacks, but only 20 percent felt the same way about Macs.

Some experts say Mac users have appeared immune to cybercrime because hackers have spent more time devising ways to hack Windows operating systems, which still run on the vast majority of PCs. As Macs grow in market share, however, they will likely receive more attention from hackers, and Mac owners should be more vigilant about keeping their computers secure by installing antivirus software and keeping their software updated, according to Alan Woodward, a visiting professor at the University of Surrey's department of computing.

"Those using Mac OS have, perhaps, been lulled into a false sense of security," Woodward wrote in a post on a blog hosted by the security firm Sophos.

He added: "Mac OS users may be 'safer' than Windows users, simply because they have fewer attacks focused on their systems, but they are not more 'secure.'"

The Flashback virus is not the first time Macs have been targeted by hackers. Last May, between 60,000 and 125,000 Mac users were infected with the so-called Mac Defender malware, a phishing scam that tried to trick users into revealing their credit card information.

And two weeks ago, security researchers found malware on older versions of Microsoft Office for Mac that was distributed by rogue emails appearing to target Tibetan activist organizations. The infected Microsoft Word documents allowed hackers to take control of Macs in order to download, upload and delete files, steal passwords and credit card numbers, or send spam.

Researchers at Intego, a security firm for Apple products, wrote that "while, in the past, we did not see this type of attack targeting Macs, it is clear that the game has changed, and that we are entering a new period of Mac malware."